Nanotechnology is commonly defined as the construction and utilization of functional structures with at least one region or characteristic dimension measured in nanometers. Such structures will be hereinafter referred to as nanodevices. The potential applications of nanotechnology are pervasive and the expected impact on society is huge. IC manufacturing technology has already arrived at sub-100 nm technology, while the fabrication of even smaller structures has already been demonstrated on a laboratory scale.
The basic building blocks of nanodevices are nanoparticles or nanostructures, such as carbon nanotubes (CNTs), quantum dots, or spherical fullerenes. A carbon nanotube comprises a graphene sheet (sheet-like structure of hexagonal network of carbon atoms) rounded into a hollow cylindrical form and can be single-walled or multi-walled. A quantum dot (or semiconductor nanocrystal) is a particle of matter in which addition or removal of an electron changes its properties in some useful way. Spherical fullerenes (also called buckyballs) are carbon molecules made up of 60 carbon atoms arranged in a series of interlocking hexagons and pentagons, forming a structure that looks similar to a soccer ball.
Assembly of nanoparticles or nanostructures into nanodevices presents significant problems, however, because the individual components or subunits are very small. Manipulation of individual nanoparticles, even when possible, is slow and tedious. There are a number of known techniques for creating various nanoparticles and for combining nanoparticles with other nanoparticles or with conventional materials to create functional nanodevices. Unfortunately, these techniques are better suited for use in laboratories than for large-scale mass-production of nanodevices.
For example, CNTs are one of the most widely utilized nanoparticles. CNTs exhibit extraordinary strength, flexibility, and unique electrical properties, and are efficient conductors of heat, making them suitable for applications ranging from AFM and STM tips, to nano-scale transistors, to reinforcing composite polymers. CNTs also make excellent field emission electron sources for use in flat-panel displays, microwave amplifiers, electron beam lithography devices, and electron microscopes.
A typical field-emitting device comprises a field-emitting assembly composed of a cathode and a plurality of field emitter tips. The device also typically includes a grid spaced relatively closely to the emitter tips and an anode spaced relatively farther from the tips. Voltage induces emission of electrons from the tips, through the grid, toward the anode. Applications include microwave tube devices, flat panel displays, klystrons and traveling wave tubes, ion guns, electron beam lithography, high-energy accelerators, free electron lasers, and electron microscopes and microprobes.
Most high-resolution electron microscopes use a specially prepared heated metal (Zr/O/W) tip as an electron source. These sources have a relatively large energy spread of the emitted electrons, which negatively impacts the resolution of the microscope. An electron source based on cold field emission has a lower energy spread. Referred to as a field emission gun, this source uses a very high electric field to pull electrons out of a very sharply pointed tungsten or other metallic tip. It is very difficult to obtain a stable electron current from such tips except in extremely good vacuum because of contamination, shape changes due to surface migration of the metal atoms, and the unavoidable bombardment by ions that are created when an electron beam is drawn from the source.
Carbon nanotubes are attractive as cold field emitters because they have the required sharp tip by nature and have shown excellent emission stability. The structure of carbon nanotubes is more resistant to ion bombardment, more resistant to contamination (because of the low sticking coefficient of the CNT structure), and can better withstand the strong electric field required for emission than metal tips. An electron beam drawn from the extremely small apex of the carbon nanotube has a high current density and a small energy spread. This can be exploited to increase the resolution of electron microscopes considerably. FIG. 1 is a line drawing of an individual carbon nanotube 202 mounted on a tungsten tip 204 and fixed in place by glue 206.
An overview of CNT electron sources is presented by de Jonge, et al. in “Carbon nanotube electron sources and applications,” Phil. Trans. R. Soc. Lond. A 362, 2239-2266(2004). CNT electron sources have been made by mounting individual carbon nanotubes directly onto a tungsten tip. Unfortunately, existing methods have a number of problems that make it difficult to reliably produce large numbers of CNT electron sources.
There are a number of known methods of producing CNTs, including carbon-arc discharge, laser ablation of carbon, or chemical vapor deposition (CVD). The CNTs are typically grown in the form of randomly oriented, needle-like or spaghetti-like agglomerates that are not easily or conveniently incorporated into individual field emitter devices. It is often difficult and time consuming to extract a single CNT for mounting. CVD can be used to grow single CNTs in a pre-determined location as described by Ren et al. in “Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot,” Applied Physics Letters, vol. 75, no. 8, 23 Aug. 1999, pp. 1086-1088, but those CNTs tend to be tightly bonded to the catalyst layer and are difficult to remove.
Further, when a single CNT is isolated, it must still be mounted on the support structure, typically by using micromanipulators and an optical or electron microscope to position the CNT and either relying on van der Waals forces alone or using an adhesive to hold the tip in place. Whatever method is used, the adhesion of CNTs to a substrate is generally a weak bond. Therefore, the CNTs have a tendency to be pulled off of the substrate at elevated electric fields. These methods often limit the maximum temperature to which the CNT can be heated, thereby limiting the post-mounting processing that can be done.
Another way to produce CNT electron sources is to grow the CNT directly on the tungsten tip by depositing a catalyst onto the tip and using CVD techniques. This method presents many challenges and difficulties and requires significant additional research before a single CNT can be grown in a repeatable fashion.
What is needed is an improved method of manufacturing certain nanodevices, including CNT electron sources, so that they can be mounted into a macroscopic device in a repeatable, reliable manner suitable for large-scale mass production.